As the service bandwidth develops rapidly, 100 G transporting technology is currently popular in an optical transport network. In order to achieve end-to-end transport of a service, presently, a 100 GB service is generally encapsulated into an OTU4, which is a 100 G OTN container (OTU4), and an OTU4 bit stream passes through a 100 G optical module and is converted into an optical signal for transport. High order modulation and coherent receiving manners are generally used to achieve high-speed and long-distance transmission.
In a process where the high order modulation and coherent receiving manners are used, generally, a transmitted electrical signal is electro-optically converted into a first transmitted optical signal Xout and a second transmitted optical signal Yout, where the first transmitted optical signal is in an X polarization state, the second transmitted optical signal is in a Y polarization state, and the Y polarization state is orthogonal to the X polarization state. The first transmitted optical signal Xout and the second transmitted optical signal Yout are combined by a combiner into one optical signal which is transmitted through an optical fiber. At a receiver end, the combined optical signal is split by a splitter into a first received optical signal X′ in and a second received optical signal Y′ in, where the first received optical signal is in an X′ polarization state, the second received optical signal is in a Y′ polarization state, and the Y′ polarization state is orthogonal to the X′ polarization state. The first received optical signal and the second received optical signal are electro-optically converted respectively into a first received electrical signal and a second received electrical signal. Original OTU4 data is then recovered according to the first received electrical signal and the second received electrical signal. In order to correctly and fully recover the original OTU4 data, the first received electrical signal and the second received electrical signal that are received at the receiver end should be respectively corresponding to the first transmitted optical signal and the second transmitted optical signal from a transmitter end.
Random birefringence may occur in the transmission of a light beam in an optical fiber, and cause crosstalk of the two orthogonal polarization states, so that the X′ polarization state and the Y′ polarization state at the receiver end include different components of the X polarization state and the Y polarization state, respectively. Therefore, the crosstalk components need to be eliminated by using an equalization method, so as to recover the original transmitted electrical signal corresponding to the X polarization state and the original transmitted electrical signal corresponding to the Y polarization state.
A commonly used equalization method is electrical equalization, where the first received electrical signal and the second received electrical signal are processed by digital filters of different coefficients (Hxx, Hxy, Hyx, and Hyy), to acquire the equalized first received electrical signal and the equalized second received electrical signal, from which the original first transmitted electrical signal and the original second transmitted electrical signal are split. An input signal for electrical equalization includes the original signal component from the transmitter end and the crosstalk signal component, and the two components are respectively present in different proportions in the input signal. The electrical equalization is characterized in that the equalized output signal is the signal corresponding to the component present in a major proportion. For example, in the case that the first received electrical signal includes the signal component corresponding to the X polarization state and signal component corresponding to the Y polarization state, where the signal component corresponding to the X polarization state is present in a major proportion and the signal component corresponding to the Y polarization state is present in a minor proportion, the equalized first received electrical signal is the first transmitted electrical signal corresponding to the X polarization state.
However, in implementing the present invention, the inventor finds that, in random birefringence occurring in the transmission of a light beam in an optical fiber, the proportions of the original signal component and the crosstalk signal component in the first received electrical signal and the second received electrical signal also change at random, so that the proportion of the crosstalk signal component in the received signal is greater than the proportion of the original signal component. Therefore, the corresponding relations of the equalized first received electrical signal and the equalized second received electrical signal output after electrical equalization to the first transmitted electrical signal and the second transmitted electrical signal at the transmitter also change at random. Accordingly, the correctness and integrity of the received signal are destroyed.